I sent the emails to exchange using smtp, worked great, but the problem was it exposed the ip and node name of our erp system to the world.

Which, IMO, means that the Exchange server isn't doing its job. If Exchange is acting as a gateway between the "inside" and the outside world, it's the task of the Exchange machine to rewrite or remove the offending headers.

So I instead sent the email through a relay machine (sendmail) which I configured to strip off the header info relating to the production machine. Worked, but mail servers have started rejecting our mail because of the relaying required to make the thing work.

That, I don't understand. Which mail servers are rejecting the mail? How do they know sendmail is relaying? Why would they care? Relaying is very common nowadays (as long as you aren't an open relay). In fact, tons of email is rejected because it's send directly, and not relayed through an ISPs SMTP server.

This is where I am stuck. When I put the message in the outbox it just sits there. When I open the message up in outlook, there is no send button. The only way I've been able send it off in the outlook gui, is to use "resend" in the tools menu.

Without knowing what you send, who could tell?

Anyone know of either a better way of doing this,

To, this sounds like a problem that needs to be solved at the MTA level.

Abigail